This invention relates to a sheet settling system, and more particularly to a method of rapidly settling a sheet on a support surface of a sheet printing or coating machine, and to a machine employing the method,
The system is applicable to printing and coating machines for sheets of metal, plastics material, paper and card.
Known sheet printing and coating machines generally include two cylinders geared together so as to contra-rotate at the same constant peripheral speed in rolling contact with each other, between which the sheets are passed successively. One of the cylinders transfers an ink impression or a coating such as varnish onto each sheet. In order accurately to register, that is to say locate in position, each sheet relative to the other of the cylinders, said other cylinder carries one or more, usually two, axially-spaced datum stops or so-called front lays for the front edge of the sheet. At least one datum stop is also provided for one side of the sheet. Each sheet is fed towards the front lays on a flat infeed surface by means such as pushers carried by endless chains or the like. When the front edge of the sheet is accurately registered against the front lays, it is gripped by a plurality of cyclically-operated grippers carried by said other cylinder and the sheet is drawn through the nip of the cylinders where it is printed or coated. As the front edge of the sheet emerges from the nip it is released by the grippers and the sheet is moved onwards for further treatment or for stacking. Because these machines operate at high speeds, there is a tendency for the sheets to move or bounce about somewhat as they are being fed towards and into contact with the datum stops, as a result of which registration is not achieved with precise and consistent accuracy.
U.S. Pat. No. 4,648,589 of Enrich et al discloses a printing press in which each sheet is supplied to a feed table where a vacuum force temporarily exerted by pivotable suction arms adjacent to the feed table retards forward movement of the sheet and is then cut off by a rotary valve (so-called suction disc), the sheet is aligned against pivotable front lays, a moveable side lay aligns the sheet against an abutment, the vacuum force is temporarily re-established by the rotary valve, and the suction arms are moved forward, so as again to press the sheet against the front lays. A first cam for pivoting the suction arms, a second cam for moving the side lay, and the rotary valve for intermittently establishing the vacuum force are all driven by, a single shaft. The front lays are pivoted by a cam which must of necessity be driven in appropriate timed relation to said shaft. This suction-assisted alignment system requires a mechanical arrangement of parts which is relatively complex.